550 Bad HELO - Host impersonating domain name

searched google, but cannot find a solution
With al hosted(mail+web) on VPS there are no problems

rDNS is set for server FQDN

Website is hosted on VPS with the p4 version of ISPConfig
The mail is hosted on an other server (hosting company, will move to VPS sometime)
DNS a records pointing to this mailserver (usin ISP DNS)
MX records pointing to this mail server

Can anyone tell me what the problem is.
what i can do, or is it a problem with the mail server from the other ISP (they say there's nothing wrong with their server)
used the perfect server setup debian wheezy with postfix (running almost a year without problems)

is this something with postfix?
there is NO email domain present in ISPConfig